How to use Separate Text Object inside a "Subprogram"?
The Separate Text Object node actually outputs a list of each individual letter of the input text object. If you want to loop into each letter you have to use the output of the separate node as input in you loop node.

From there you can do any modifs to the letter you want. What you where trying to do could be implemented like this, and there is no need to nest loops.



If you get confused by the Object Id Key node: Object Initial Transforms, try to think about what would happen if was a single vector. All the letters would animate to the same position (as long as they have an individual origin).
The Object Initial Transforms takes the inital position of each letter and outputs the vector, so you can properly animate the text.

If you want to animate individual letters of mutliple text objects, there is no solution yet apart from making two different loops of the separate node. You can implement the group functionnality which is very useful when you have to repeat a process like there.





Maybe an input for the seperate individual letters will be implemented in future realeses.
Note that Jacques Luke now include a very nice Animate individual Letter template, which is basically the same node setup than the first image, but animating matrices.